3.9 misfire

Silva · Post by **Silva** » Wed Dec 04, 2013 9:17 pm

Hi all! Got a 3.9 on carbs in a stock car which has a misfire when drove at around 4000 revs. Revs beautifully if car is stationary. But is terrible when driven! Any pointers in the right direction is gratefully welcomed! Thanks

ChrisJC · Post by **ChrisJC** » Wed Dec 04, 2013 10:03 pm

What ignition system?

Chris.

Silva · Post by **Silva** » Thu Dec 05, 2013 7:44 am

its the one with the amplifier with the coil mounted on it pal

DaveEFI · Post by **DaveEFI** » Thu Dec 05, 2013 9:14 am

Silva wrote:its the one with the amplifier with the coil mounted on it pal

Not a new dizzy off Ebay, is it?

Silva · Post by **Silva** » Thu Dec 05, 2013 9:55 am

no sir! at the mo its on its origional gear, the engine has just been refreshed and the carbs worked fine on another engine, so im thinking its electrical.

DaveEFI · Post by **DaveEFI** » Thu Dec 05, 2013 11:19 am

First thing to check with that sort of fault is the plug leads - assuming the plugs are ok. And don't forget the king lead to the coil.

Silva · Post by **Silva** » Thu Dec 05, 2013 12:51 pm

plugs and leads are new pal! also forgot to mention,dont know if its a related problem but the revs on tickover go up and down between 800 and 1100 revs

DaveEFI · Post by **DaveEFI** » Thu Dec 05, 2013 1:54 pm

Is the mixture correct?

Silva · Post by **Silva** » Thu Dec 05, 2013 4:18 pm

i would hope so i had a guy set the carbs for me! he might have done it wrong tho!how do i adjust the mixture? sorry for sounding numb! but im completely new to v8 stuff! cheers

kiwicar · Post by **kiwicar** » Thu Dec 05, 2013 7:23 pm

Hi
when you say carbs do you mean SUs? if so then solder the poppet valves in the throttle plates shut, and reset the mixture as per the hains manual using the lifter pins on the carbs, the 800 - 1100 revs cycling is a classic symptom of the springs poppet valves having gone soft over the years.

best regards
Mike

ChrisJC · Post by **ChrisJC** » Thu Dec 05, 2013 7:46 pm

Have you got a spare coil / amp block that you can swap out?, or distributor / amp / coil set? That is a quick way to confirm it is ignition related.

Chris.

Silva · Post by **Silva** » Thu Dec 05, 2013 7:46 pm

Yes sir! I do meanSU carbs. I will solder them tomorrow if that will cure the fluctuation in tick over revs. As for the misfire I can only tell if I've cured it when I take it for a test run down a private road! Does anyone know somebody in the manchester area who is good with v8s?

DaveEFI · Post by **DaveEFI** » Thu Dec 05, 2013 11:34 pm

Nothing special about a V8. But you may need someone familiar with SU carbs - especially if they are not the factory fit ones.

ChrisJC · Post by **ChrisJC** » Fri Dec 06, 2013 5:56 pm

This brings to mind a problem I had years ago. Similar symptoms. The issue was the wrong ignition coil for the amplifier.

Chris.

Silva · Post by **Silva** » Fri Dec 06, 2013 7:57 pm

Gona go for a test run on the private road tomorrow! Will let you helpful guys know how I get on! And thanks for the pointers lads!